Silicon continues to play an important role in many technologies including CMOS, microelectromechanical systems (MEMSs), and wafer level packaging, as well as a substrate for emerging technologies such as piezoelectrics. One of the fundamental aspects which makes silicon so advantageous is the ability to sculpt bulk silicon to create structures using deep reactive ion etching (DRIE) techniques. Plasma etch techniques for the rapid etching of silicon, with etch rates above 1 μm/min, include cryogenic etching and time multiplexed processes where SF6 provides substantial improvements over Cl2- or HBr-based etches. One of the key aspects for successful device fabrication using fluorine-based DRIE is the utilization of etch masks which have a high etch mask selectivity relative to silicon. High selectivity in etch masks reduces undesired artifacts in etch profiles such as sidewall tapering, while also enabling the use of thinner etch mask films. Minimizing the etch mask thickness reduces the amount of wafer bow induced by film stress and results in a shorter deposition time. Thus, there is a need to develop higher selectivity etch masks to enable new technology with more difficult plasma etching requirements.
Etch mask selectivity is typically controlled by two major etching mechanisms in plasma etch techniques: chemical etching and physical sputtering. Utilization of fluorine-based chemistries offers significant etch mask selectivity improvement since silicon and germanium are energetically favorable to react with fluorine to produce volatile silicon tetrafluoride (SiF4) and germanium tetrafluoride (GeF4). Etch masks which do not chemically react with fluorine, such as Cr, Ni, Al, Ga, and Al2O3, have higher selectivity to silicon than those that can react with fluorine to produce volatile compounds, such as Ta, Nb, Ti, and W. For nonreactive etch mask materials, neutrals and radicals in the etching processes are less important to etch mask selectivity than ion impingement.
One approach to improving the selectivity of nonreactive etch mask materials is to reduce the ion impingement. DRIE techniques can be tuned to have low impingement chemistries that only lightly accelerate the plasma created ions, reduce the ion flux, and dramatically reduce the sputter yield of the etch mask. A lower sputter yield (γs) and lower ion flux (jion) dramatically reduce the etch mask erosion rate (dZ/dt), given in Eq. (1):
                                          d            ⁢                                                  ⁢            Z                                d            ⁢                                                  ⁢            t                          =                              W                          ρ              ⁢                                                          ⁢                              N                A                            ⁢              e                                ×                      γ            s                    ×                      j            ion                                              (        1        )            where W and ρ are the atomic weight and density of the etch mask material and NA and e are Avogadro's constant and electron charge. Using sputter yield values of 1.34, 1.34, and 0.18 for Ni, SiO2, and Al2O3, a reduction of etch mask etch rate per ion current density [(dZ/dt) per jion] can be predicted with values of 9.3×10−5, 3.6×10−4, and 4.8×10−5, respectively, yielding roughly a factor of 8 improvement when utilizing Al2O3 in place of SiO2. This improvement has been experimentally measured multiple times, enabling deeper and higher aspect ratio structures in silicon. With fluorine-based etch chemistries, reaction products for Ni and Al containing etch masks are nonvolatile NiF and AlF, respectively. This is helpful for etch mask selectivity, but it results in micromasking of silicon due to etch mask sputtering and redeposition of the etch mask material and its nonvolatile reaction products.
Micromasking results in unwanted spikes of silicon as the etch progresses, as shown in FIGS. 3A and 3B, which illustrate micromasking from the use of prior art Al2O3 and AlN etch masks, respectively. Although all etch masks have some amount of sputtering, a wide range of sputtering characteristics exist based on the etch mask material, etch chemistry, and etch bias. Typically, to compensate for etch mask sputtering, plasma etch bias is reduced, which consequently reduces both the etch mask sputtering and the silicon etch rate.
Thus, there exists a need for an etch mask for DRIE that would be chemically nonvolatile in fluorine-based etch chemistries while also having a low sputter yield and creating a high etch mask selectivity relative to silicon and other semiconductor materials. This low sputter yield of the etch mask would also reduce the micromasking effect.